In an ordinary double twister, a yarn unwound from a yarn supply bobbin supported on a stationary disc is guided into the central hole of the yarn supply bobbin from the top of the central hole of the bobbin, passed through a tension device such as a capsule tensor or ball tensor, travelled from the central hole of a spindle to the center of a yarn storing disc and delivered to the outside of the yarn storing disc through a yarn guide hole extended from the center of the yarn storing disc in the radial direction. Then, the yarn is wound on the peripheral surface of the yarn storing disc at a certain winding angle and is raised on the outer wall surface of the yarn storing disc, and the yarn is separated from the top end portion of said wall surface and then delivered upward while being ballooned. Then, the yarn is passed through a yarn guide located on an extension of the central line of the bobbin and is wound by a winding device disposed above.
When pneumatic yarn passage is carried out in such double twister, it is necessary to form an air circulation passage and produce a sucking force in the yarn guide hole by jetted air. However, in the tensioning zone comprising the above-mentioned tensor such as a capsule sensor or ball tensor and a yarn contact guide supporting the tensor thereon, the air circulation passage is intercepted, and therefore, a sucking force cannot be produced in this zone. Accordingly, when pneumatic yarn passage is carried out in the above-mentioned ordinary double twister, it is necessary that the tensor should be separated from the center of the opening of the yarn contact guide, and at the time of yarn passage, it is necessary that the tensor should be deviated from the center of the yarn guide hole by using a tensor escape member and air should be jetted while the tensor is maintained in this deviated state. Accordingly, the operation is complicated and troublesome.
Furthermore, in an ordinary double twister, the balloon tension is always maintained at a certain level, and as factors determining this balloon tension, there can be mentioned, for example, the rotation number and diameter of the yarn storing board, the fineness of the yarn to be treated, the air resistance, the tension applied by the tensor, the friction of the yarn wound on the outer surface of the yarn storing board and the unwinding tension on the yarn unwound from the yarn supply bobbin.
Among these factors, the rotation number and diameter of the yarn storing disc, the air resistance and the tension by the tensor are fixed in one double twister, and as variable factors, there can be mentioned the unwinding tension and the winding angle of the yarn on the yarn storing disc.
More specifically, the larger is the diameter of the yarn supply bobbin, the smaller is the unwinding tension, and as the yarn is unwound and the diameter of the yarn supply bobbin is reduced, the unwinding tension is increased. With this variation of the unwinding tension, the winding angle of the yarn on the yarn storing disc is changed, whereby the balloon tension is maintained at a constant level. More specifically, with increase of the unwinding tension, the winding angle is decreased and the contact length of the yarn is decreased, with the result that the friction is decreased and a certain balloon tension is consequently mantained. In this connection, it must be noted that the yarn is wound at a certain angle on the peripheral surface of the yarn storing disc until the yarn on the bobbin is wound to such extent that no yarn is left on the bobbin. If the winding angle of the yarn is reduced to zero, variations of the unwinding tension cannot be absorbed in the yarn storing disc any more and yarn breakage takes place. On the contary, if a tensioning device such as a tensor is not disposed, the yarn is wound by several turns on the yarn storing disc, and yarn breakage takes place also in this case.
Therefore, in an ordinary double twister, a spring of a capsule tensor built in the central hole of the bobbin is adjusted in advance, or in case of a ball tensor, the number of balls is increased or decreased in advance, and the tensor tension is thus adjusted so that the yarn is wound on the yarn storing disc at a certain angle up to completion of the winding operation while the winding angle is being changed by changes of the unwinding tension on the yarn.